This invention relates to gage glasses used in liquid level gages and sight glasses which are used to view the interior of pressure vessels, and more particularly to gage glasses that are adapted to resist corrosion and erosion by the contents of the gage or vessel, to withstand variations in temperature and pressure and to resist leakage even with the glass may become cracked or broken.
Gage glasses are used in liquid level gages and sight glasses to allow visual observation of liquids in a container or pressure vessel. Liquid level gages comprise a vertically elongated chamber in fluid communication at both the top and bottom with a liquid chamber such as a boiler, process tank, or storage tank. The provision of one or more gage glasses along a wall of the chamber allows a visual observation of the liquid level in the chamber. If a pair of glasses are arranged on opposite sides of the gage chamber, the level can be observed by transmitted light, while a glass in a single wall may require a prism surface adjacent the liquid to allow observation by reflected light. In both cases, the exposure of the surface of the glass to the liquid may allow corrosion or erosion of the glass surface to a point where the unit is no longer functional unless the gage glass is replaced, and such replacement usually involves disassembly of the unit.
Sight glasses use a single gage glass and have long been used to allow visual examination of the interior of certain vessels or containers under a wide range of conditions. A sight glass must employ a gage glass which is large enough, depending upon the application, to allow adequate visual inspection of the interior of the container and must be made resistant to the physical and chemical forces that tend to damage the sight glass. For example, the glass may be exposed to various temperatures and pressures which apply high stresses to the glass, and it may be subject to chemical corrosion depending upon the contents of the vessel. The result of these forces acting on the glass tends to make it subject to breakage and subsequent leakage of the contents, and also chemical attacks may make the glass become opaque so that it must be replaced with a clear glass to allow proper visual inspection. Thus, the sight glass must be mounted to provide adequate sealing against leakage, and must also be mounted in such a way that it can easily and quickly be replaced if it becomes damaged.
While in the beginning single prisms of glass, usually circular in shape and having parallel front and rear sides, were used, it has been discovered that the use of two or more separate glass lenses in a suitable mounting provides better resistance to breakage. It has also been discovered that when the glass lenses are subjected to compressive forces, their strength is greatly increased and it is possible to have one of the lenses crack without resulting in any leakage. Furthermore, in order that replacement may be done quickly and easily with a minimum of downtime, the glass has been mounted as a subassembly on a flange with a suitable cover so that the placement can be done simply by unbolting the cover, inserting a replacement subassembly, and then refastening the cover in place on the flange.
Other improvements made in such sight glasses include the bonding together of the separate glass lenses by plastic laminating material, the use of a plastic laminating material as a sealing agent around the outer periphery of the lenses between the lenses and a metal shell which supports them, and the use of chemically resistant materials on the inner facing of the glass lens for protection of the glass against chemical attack by the contents of the vessel.